Plastic coatings for articles and a method of producing same

ABSTRACT

A METHOD OF PRODUCING A PLASTIC COATING ON AN ARTICLE IS DISCLOSED WHICH CONSISTS IN ARRANGING THE ARTICLE AS THE ANODE IMMERSED IN AN ELECTROLYTE IN AN ELECTROPHORETIC CELL, THE ELECTROLYTE INCLUDING A CODISPERSION OF THE PLASTIC AND A FINELY DIVIDED SOLID, PASSING AN ELECTRIC CURRENT THROUGH THE CELL UNTIL AN ADEQUATELY THICK LAYER OF THE INTERMIXED PLASTIC AND SOLID HAS BEEN DEPOSITED ON THE ARTRICLE, DRYING AND THE DEPOSITED LAYER AND HEATING THE LAYER UNTIL IT FUSES OR COALESCES.

United States Patent 3,788,961 PLASTIC COATINGS FOR ARTICLES AND A METHOD OF PRODUCING SAME Alan Buxton, Leamington Spa, England, assignor to Associated Engineering Limited, Leamington Spa, England No Drawing. Filed Apr. 30, 1971, Ser. No. 139,177 Claims priority, application Great Britain, May 11, 1970, 22,741/ 70 Int. Cl. B01k 5/02; C23b 13/00 US. Cl. 204-181 4 Claims ABSTRACT OF THE DISCLOSURE This invention relates to a plastic coating for an article, to an article coated with such a coating and to a method of producing such a coating.

It is known to produce a coating of a plastic, such as a polymer of tetrafluorethylene, on an article by electrophoresis in which the article is the anode and the electrolyte includes a co-dispersion of the plastic and a film-forming polymer. The deposited film is subsequently dried and heated to a temperature at which the plastic fuses or coalesces. At this temperature the film-forming polymer will have evaporated or decomposed. It has been found that a plastic coating formed by such a process has a tendency to crack and curl up.

According to one aspect of the present invention, a method of producing a plastic coating on an article includes arranging the article as the anode immersed in an electrolyte in an electrophoretic cell, the electrolyte including a co-dispersion of the plastic and a finely divided solid, passing an electric current through the cell until an adequately thick layer of the intermixed plastic and solid has been deposited on the article, drying the article and the deposited layer, and heating the layer until it fuses or coalesces. The plastic may be a polymer of tetrafluorethylene such as polytetrafiuoroethylene.

One suitable finely divided solid is finely divided asbestos.

According to another aspect of the invention, an article is provided with a plastic coating produced by electrophoresis of an electrolyte including a co-dispersion of the plastic and a finely divided solid.

On passing the usual D.C. electric current through the electrophoretic cell a layer will be deposited on the article acting as anode. The layer Will be a mixture of the polytetrafiuoroethylene and asbestos. When a suitable layer thickness has been reached the article is removed and the layer is dried in some appropriate manner. The article and layer are then heated to a temperature around 330 C.400 C. at which the polytetrafiuoroethylene fuses or coalesces to form a continuous and stable layer. It has been found that 25% by Weight of asbestos in the final layer is a satisfactory quantity but other percentages may be found appropriate, especially if other plastics or finely divided solids are used.

It has been found that the size of finely divided particles causes them to settle out in time, and the term co-dispersion is intended to include finely-divided particles of such size, in addition to particles which remain ice.

substantially permanently in suspension in the electrol te.

y The invention will now be further described, by way of example.

The article to be coated with the plastic coating is mounted in an electrophoretic cell of known kind. The article is connected as the anode in the cell of which the cathode is conveniently iron or copper immersed in a sodium chloride solution in a porous pot. The electrolyte in which the anode is immersed depends on the type of coating to be deposited thereon but contains, in co-dispersion, the plastic and the finely divided solid. An electrolyte consisting of 10 grammes asbestos, grammes water and 25 cc. PTFE dispersion has been used. This electrolyte has also been used with the addition of 53 grammes lead oxide. There has also been added sufficient tin bronze to the electrolyte containing lead oxide to produce 10% by weight of tin bronze in the final coating. Alternatively, by omitting the lead oxide suflicient tin bronze has been used to produce 54% by Weight of bronze in the final coating.

The asbestos is in finely divided form and the bronze is ball-milled to pass through 300 mesh. A suitable PTFE dispersion is known as Sorefion 60 Type 3 (registered trade mark). Deposition can be effected on any electrically conducting surface such as steel, which it may be advantageous to grit-blast before deposition. In some instances, it may be desired to copper plate the surface of the anode before deposition, for example to prevent corrosion of a steel article through pin holes in the deposited coating or anodization of an aluminium article.

The deposition process has been conducted at room temperature. It has been found that a DC. current density of .05 amp per square inch of the anode produce a satisfactory coating with a deposition rate of approximately 0.001 of an inch per two seconds and coating thicknesses of 0.015 of an inch have been produced.

When deposition is completed the article is removed from the electrolyte and dried in a hot air current or an oven at C. When dry, the coating is sintered in a mufile furnace at 380 C. in air.

In addition to a coating per se the deposited coating can be used as a bonding layer for bonding ordinary PTFE to steel. In this process the air dry electrocoating is covered with PTFE powder which is pressed, the pressure removed and then sintered. By a similar process bronze filled PTFE can be bonded onto steel.

It is important that the solid material in co-dispersion in the electrolyte is in finely divided form. Other fibrous solids than asbestos may be used, especially if the fibres are of dendritic type. Instead of asbestos, very fine platelets of aluminium have been used in which the thickness is very much less than the width or length.

This process is particularly but not exclusively applicable to the deposition of polymers of tetrafiuoroethylene such as polytetrafiuoroethylene.

I claim:

1. A method of producing a polytetrafiuoroethylene coating on an article which includes arranging the article as the anode immersed in an electrolyte in an electrophoretic cell, the electrolyte consisting essentially of a co-dispersion of the polytetrafiuoroethylene and at least one finely divided solid, one of which finely divided solids is fibrous, passing an electric current through the cell until an adequately thick layer of the intermixed polytetrafiuoroethylene and at least one solid has been deposited on the article, drying the article and the deposited layer and heating the layer until it fuses or coalesces.

2. A method as claimed in claim 1, wherein the finely divided fibrous solid is powdered asbestos.

3. A method as claimed in claim 1, wherein the finely divided fibrous solid is of the dendritic type.

4. A method as claimed in claim 1, including the addi- 2,865,795 12/1958 Morrison, Jr 204181 X tional step of bonding an additional layer of the plastic 3,434,942 3/1969 Waterman 204--38 E material to the plastic coating and wherein the dry plas- 3,449,230 6/1969 Heron et a1. 204180 R tic coating is covered with a powder of the plastic mate- 3,702,813 11/1972 Tanaka et al. 204181 rial which is pressed, the pressure removed and then sin- 5 3,445,361 5/1969 Sicka et al. 204--181 tered. 3,626,041 12/1971 Fields et al. 204-108 R X References Cited UNITED ST P JOHN Primary Examiner 2,478,322 8/1949 Robinson et a1 204-481 PRESCOTT, 4881mm Exammer 2,707,703 5/1955 Dorst 204-38 E X 10 2,800,447 7/1957 Graham, Jr. 20-4 1s1 2,820,752 1/1958 Heller z04 1s1 

